Perl's regex engine doesn't search from the end, but you can anchor your match at the end to make it backtrack. See perlre. 

perl -Dr -e '
  @l = ("A".."Z","a".."z",qw(< / >),0..9);
  $_ = join( "", map { $l[ rand @l ] } 1 .. 100_000 )
     . "</body></html>";
  s|(</body></html>)$|zzz$1|

'
Compiling REx "(</body></html>)$"
rarest char < at 0
Final program:
   1: OPEN1 (3)
   3:   EXACT <</body></html>> (8)
   8: CLOSE1 (10)
  10: EOL (11)
  11: END (0)
anchored "</body></html>"$ at 0 (checking anchored) minlen 14 
Omitting $` $& $' support.

EXECUTING...

Guessing start of match in sv for REx "(</body></html>)$" against "Fw3
+EK>>Y6x<q>x4s7sACk6xtpG9etod1O4uibfbVBwJNJLRurKYn>SdVRt2o"...
Found anchored substr "</body></html>"$ at offset 100000...
Starting position does not contradict /^/m...
Guessed: match at offset 100000
Matching REx "(</body></html>)$" against "</body></html>"
100000 <yUnOt> <</body></h>    |  1:OPEN1(3)
100000 <yUnOt> <</body></h>    |  3:EXACT <</body></html>>(8)
100014 <body></html>> <>       |  8:CLOSE1(10)
100014 <body></html>> <>       | 10:EOL(11)
100014 <body></html>> <>       | 11:END(0)
Match successful!
Freeing REx: "(</body></html>)$"

[download]

As you see, the anchor will be checked first, and then the rest is quite inexpensive. Small benchmark... 

use Benchmark qw(cmpthese);
@l = ("A".."Z","a".."z",qw(< / >),0..9);
$x = join( '', map { $l[ rand @l ] } 1 .. 100_000 ) . "</body></html>"
+;

cmpthese ( -1,
  {
    noanchor => sub { local $_ = $x; s|(</body></html>)|zzz$1| },
    anchor   => sub { local $_ = $x; s|(</body></html>)$|zzz$1| },
  }
);
           Rate noanchor   anchor
noanchor 5023/s       --     -19%
anchor   6222/s      24%       --

[download]

In reply to Re: Optimizing a regex that replaces near the end of a string by shmem
in thread Optimizing a regex that replaces near the end of a string by perl5ever

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